1. Field of the Invention
This invention relates broadly in the field of electronic musical tone generators and in particular is concerned with apparatus for producing tones with time variant spectral characteristics with nonharmonic overtones varied in response to a loudness control signal.
2. Description of the Prior Art
An elusive goal in the design of electronic musical instruments is the ability to realistically imitate the sounds produced by the familiar family of conventional acoustic type musical instruments. The best results have been obtained for electronic musical instruments which imitate wind-blown organs and harpsichords. The principal reason for obtaining good imitative results for these instruments is that they are essentially mechanical tone generators. The tone generation is automatic and the musician only actuates on-off switches. With the notable exception of these two instruments, the tone character of almost all other musical instruments is a function of certain skills possessed by the musician.
It has long been recognized that with the exception of conventional organ tones, almost all tone produced by musical instruments exhibit tone spectra which are time variant. The recognition of the characteristic time variant spectra has motivated the development of electronic music generating systems such as those known by the generic names of "sliding formant" and "FM-synthesizer." Sliding formant tone generators constitute a class of generators which are also called subtractive synthesis. In subtractive synthesis, the fundamental tone source generates more than the desired tone spectral components and the undesired spectral components are attenuated, or filtered out, by means of some variety of frequency filter. The FM-synthesizer is of the additive variety in that FM (frequency modulation) is used to add components to a source signal which often consists of a simple single frequency sinusoid time function.
Part of the problem in trying to synthesize musical instrument sounds rests in the inability to adequately model many of the subtleties in tone structure imparted by the musician in an effective performance on his instrument. The musician commonly uses a technique such that the tonal structure for a given musical note varies with the loudness of the tone. Repeated notes are played with different loudness and tonal structures. It is these subtle differences that differ from the mechanical-like repeated tones produced by most electronic musical tone generators. Even very expert players are unable to repeat a given tone with precisely the same identical tone spectra. In general, as the tone level becomes louder, the tone spectra increases in the number and strength of the higher harmonics. Very soft tones tend to approach tones having only a few harmonics.
To compound the task of imitating musical instruments is the fact that even a sustained tone does not have a constant tone spectra. For almost all instruments it is well-known that the spectrum will change with time and change with the tone's instantaneous amplitude envelope. Such time variant spectralal variations have been introduced into electronic musical instruments using synthesis techniques such as the sliding formant and FM-synthesizer.
In the copending patent application Ser. No. 139,908 filed Apr. 14, 1980 entitled "A Polyphonic Tone Synthesizer With Loudness Spectral Variation" a system is disclosed whereby a loudness control system is used to vary the spectral content of generated musical tones. The referenced patent application and the present invention are assigned to the same assignee. The invention disclosed in the application describes means for causing a tone generator of the type described in U.S. Pat. No. 4,085,644, entitled "Polyphonic Tone Synthesizer," to produce musical tones having spectral components which can be made to be time variant in response to a control signal such as the note's ADSR envelope function or the setting of a loudness control signal.
In a Polyphonic Tone Synthesizer of the type described in the referenced patent, a computation cycle and a data transfer cycle are repetitively and independently implemented to provide data which are converted into musical waveshapes. During the computation cycle a master data set is generated having a spectral content which is variable in response to an input loudness control signal. This is accomplished by utilizing a table of stored sinusoid values to address data values from a transform memory which contains a preselected set of data points. The sinusoid function values are scaled in magnitude in response to the input loudness control signal thereby causing a variable subset of the transform memory contents to be read out and stored in a main register. These operations are executed at a fast rate which may be nonsynchronous with any musical frequency.
Following a computation cycle, a transfer cycle is initiated during which the master data set is first adjusted to have a zero average value and is then transferred to selected members of a multiplicity of note registers. The data residing in the note registers are read out in response to note clocks having frequencies corresponding to the assigned corresponding keyboard switches. This data is read out sequentially and repetitively and converted to analog musical waveshapes. Tone generation continues uninterrupted during the computation and the transfer cycles.
The usual method of producing time variant spectral changes is to use the sliding formant technique. These techniques are usually implemented by using a frequency-domain filter with controllable cut-off frequencies to process waveshapes generated by analog musical instruments.
A different type of sliding formant tone generation is described in U.S. Pat. No. 3,908,504 entitled "Harmonic Modulation And Loudness Scaling In A Computor Organ" and in the previously referenced U.S. Pat. No. 4,085,644. In these systems the time variation of spectral content is obtained by varying the magnitude of harmonic coefficients used to compute waveshape amplitude points by means of a Fourier-type algorithm. The tonal effects obtained by "sliding" the magnitude of selected harmonic coefficients are very similar to tonal effects obtained by "sliding" the cut-off frequencies of a frequency domain filter.
In the copending patent application Ser. No. 150,493 filed May 16, 1980 entitled "Apparatus For Tone Generation With Combined Loudness And Formant Spectral Variation" a system is disclosed whereby the loudness control spectral variations produced by the system disclosed in the referenced application Ser. No. 139,908 are combined with a means for producing spectral variations of the sliding harmonic type described above. This is accomplished in a tone generator of the Polyphonic Tone Synthesizer type by partitioning the computation cycle into two segments. During the first segment of the computation cycle a set of transform data values are computed and stored in a transform memory. These values are computed using a generalized Fourier-type algorithm having a preselected set of harmonic coefficients as input data. These harmonic coefficients are scaled in magnitude by means of a formant scaling subsystem to produce transform data values which are varied in spectral content in response to the control signals applied as input data to the formant scaling subsystem. During the second segment of the computation cycle, the data residing in the transform memory are used to create a master data set in a manner analogous to that disclosed in the referenced patent application Ser. No. 139,908. In this fashion the tonal effects of a time varying formant is combined with that of a time varying loudness control.
An application of nonlinear system transformations in the generation of musical sounds is contained in the technical article: Beauchamp, J., "Brass Tone Synthesis by Spectrum Evolution Matching with Nonlinear Functions." Computer Music Journal, Vol. 3, No. 2, pp 35-42 (1979).
To imitate certain percussive musical sounds such as chimes and bells it is necessary to generate tones that contain nonharmonic overtones. The FM-synthesizer is one method of generating such tones. Another method is to employ the tonal effect used in musical tone synthesizers which is generically called "ring modulation." In a ring modulator two signals at different frequencies are combined in a manner such that the output signal contains spectral components at the sum and difference frequencies of the original two frequencies. A description of a variety of ring modulators is described in U.S. Pat. No. 4,135,427 entitled "Electronic Musical Instrument Ring Modulator Employing Multiplication Of Signals."